Thermostatic control for electrical heating elements



March 7, 1950 G. w. CRIS: 2,499,906

THERMOSTATIC CONTROL FOR ELECTRICAL HEATING ELEMENTS Filed July 23, 1947 INVENTOR.

Ge orge W. Crz'se BY A TTORNE Y.

Patented Mar. 7, 1950 UNITED STATES PATENT OFFICE,

THERMOSTATIC CONTROL FOR ELEC- TRICAL HEATING'ELEMENTS 8 Claims.

1 The present invention relates to thermostatic switches, and has particular reference to an improved thermostatic control for electrical heating elements and the like. In its more specific aspects, the invention deals with a thermostatic switch control adapted for use in conjunction with electrical bed warmers and blankets.

The principal object of the present invention is to provide a simple and mechanically efficient thermostat for controlling the operating temperature of an electric bed warmer or blanket,

which functions in a safe and eiiicient manner to scribed which comprises but few and relatively simple component parts which may be assembled within a relatively small and compact unit, and as such, occupies but little space, and may be positioned remotely of the bed warmer or blanket, thereby eliminating undesirable and uncomfortable bulges within the associated bed warmer.

For a further and more detailed understanding of the present invention and the additional objects and advantages thereof, reference is made to the following description and the accompanying drawing, which discloses in diagrammatic form the electrical cincuit and component parts utilizedin the present invention.

Referring now to the drawing, the present invention makes use of a pair of independently supported bimetallic arms and 6, with the arm 5 being rigidly attached at its inner end to a stationary insulated support 1, and the, arm 5 being rigidly secured to a pivotally mounted insulated supportfl. Both of the arms 5 and 6 are positioned upon their respective supports so as to deflect downwardly, as indicated by dotted lines in the drawing, in response to the application of heat thereto, it being noted that both of the arms will deflect in the same general direction. Carried at the outer end of each of the arms 5 and 5 are a pair of relatively engageablecircuit contact points 9 and Ill. The arms 5 and 6 and their associated contacts arenormally positioned so. as to maintainthe contacts in engagement with one 2 another, and in the event the arms 5 and 6 are thermally deflected in an even and uniform manner, and to the same extent, the contacts 9 and Hi will remain in sliding but continuous engagement with one another.

Disposed in proximity to the deflectable arms is an insulated circuit terminal support H provided with circuit terminals l2, l3 and [4. Connected with the circuit terminals l3 and Marc lead wires 15 and 16 respectively, which serve to furnish current for the switch and its associated load. Electrically connected with the terminals (2 and ['3 is a main heating element H which is normall positioned within the bed warmer or blanket, and which comprises a coil of resistance wire,.such.as copper, or alloys thereof, and which is of relatively high resistance, with its resistance increasing as the temperature rises within the coils. Leading from the terminal 14 is a lead wire [8 which is electrically joined with a low resistance. heating coil l9 wound about the bimetallic arm 6. The opposite end of the low resistance coil 19 is electrically joined through the arm '6 to its associated contact point 9. Upon engagement of the contact points 9 and ID, the circuit continues through the contact Hi and its associated arm 5 to a second lead wire 20 joining the inner end of the bimetallic arm 5 with the circuit terminal 12. In this manner, it will be seen thatcurrent is furnishedthe-main heating r which is connected at its respective ends to the bimetallic arm 5 and to the circuit terminal 13 by means of a third lead wire 22. Thus it will be seen that both the main heating element I! and the relatively high resistance heating coil 2! are connected together in parallel, whereby both operate'from substantially the same voltage. Preferably, the low resistance heating coil 19 is formed from any suitable alloy having constant resistance characteristics or a negative coeflicient of. resistance, whereby the resistance of the coil remains substantially unchanged with an increase or decrease in' temperature. One suitable alloy answering these specifications is known commercially as manganin, and is an alloy of copper, nickel and manganese. The relatively high resistance heating coil 2| may be formed from any suitable high resistance alloy, such. as Nichromeor the like, and need not possess a substantially. constant resistance or coefiicient. of're'sistance. As before stated, the main and it, will be warmed and will be maintainedv at a substantially constant temperature, andby 4 24. The bimetallic arm 23 is thermally insulated from the heating coils I9 and 2! by means of a barrier wall 25 formed from any suitable thermally insulating material which will serve to prevent the passage of heat by convection from the heating coils to the bimetallic arm 23, whereby the same will act only in response to temperature variations within the atmosphere surjrounding the switch. The arm 23 is positioned thermal conductivity, its associated bimetallic arm 6 will be warmed and deflected downwardly, as indicated by broken lines in the drawing. At the same time, current will pass from the coil i9 through the contacts 9 and I0, and through the bimetallic arm 5, by way of the second lead wire 29, to the circuit terminal 12 and thence to the main heating coil il, whereupon the circuit is completed withthe supply conduit l6 thereby heating the main heating coil H. As

before stated, the relatively high resistance heating coil 2| is connected in parallel with the .main heating element l1, and thereby receives the same operating voltage, and will be warmed by the same voltage as the 'main heating element ll. the bimetallic arm 5 causing the same to deflect In this manner, heat is supplied to downwardly, as is also indicated by dotted lines in the drawing. supplied by the heating coils I9 andZl will be substantially the same, with the two bimetallic arms f, deflecting downwardly at a uniform rate, and to Initially, the amount of heat .-the, same extent, but as the mainheating element I1 is warmed, its resistance will be increased and will therefore pass a decreased operating current, thus diminishing the operating current fjwithin the relatively low resistance coil l9.

At the same time, the high resistance coil 2|,

being in parallel with the main heating element, will receive the same voltage as the heating element, and consequently will become heated to a slightly greater extent due to line voltage increase resulting from load decrease thereby causing the bimetallic arm 5 to deflect a'slightly greater distance downward while at, the same time the reduced current being, drawn through contact points 9 and Hi will again become engaged to repeat the cycle of operationfjust described. It will be understood, that 'thevarivation in the extent of deflection ofthe two bimetallic arms 5 and Bis accomplished wholly through the differenceintemperature. of the two heating coils l9 and 2!, the same. remaining substantially. constant until an increase. in

temperature is accomplished withinthe rnain heating element 11, whereupon its resistance in; creases, causing a slight increase in the heat sup ply of the heating' coil 2i and ,a substantial. decrease in the heat supply ofcoil l9 To vary the tension between the. contact points S! and It independently of the thermal deflection, ofthe arms 5 and'6, a third bimetallic arm 23' is rigidly secured atone of its ends to the opposite side of the pivotally mounted insulated supf p ort ,8 ,The oppositeen'd Qfithis bimetallica'rm carried by and 'subjected'to the movement of a manually controlled and pivotally mounted lever so as to deflect downwardly, as indicated by broken lines in the drawings, in response to an increase in temperature of the surrounding atmosphere.

Thus, as the temperature of the surrounding atmosphere increases, the arm 23 will be defiected downwardly and will cause the pivotally mounted support 8 to rock in a limited clockwise motion, as viewed in the drawing. This rocking motion within the support 8 will, in turn, tend to lift the bimetallic arm 6 upwardly, and will tend to decrease the tension between the contact points 9 and i0, thereby decreasing the amount of relative deflection between the arms 5 and 6 necessary to open the power circuit of the switch. In addition to the thermostatic control provided by the independent bimetallic arm 23, the tension between the two contact points 9 and ill may be varied by the movement of the manually adjustable lever 24, and as the same is rotated or pivoted in a substantially counterclockwise manner, the tension between the points will be decreased causing an opening of the operating circuit at a relatively lower temperature within the heating element H.

To prevent overheating within the heating elesion between the contact points 9 and H] and limiting the clockwise movement of the manually operable lever 24 to a desired maximum safe an initial resilient tension within the deflectable arms 5 and 6, as the same are arranged as shown atmosphere.

in the drawing, this tension may be decreased by action of the bimetallic arm 23 acting in response toa temperature increase of the surrounding room In addition, the manually adjustable lever 24 may be adjusted to control the .degree of tension within the two arms 5 and 6 in accordance with the desires for a predeter- 'fimined maximum temperature within the main ,heating element I1, thereby controlling the maximum temperature which the heating element I! Will reach before the opening of the circuit con tacts 9 and Ill.

In view of the foregoing, it will be seen that the present invention provides both a highly useful and mechanically efiicient thermostatic switch for controlling the operation of an electrical heating element, with particular reference to electrical heating elements of the type utilized in bed warmers or electrically heated blankets.

The switch operates efiiciently to control both the operating temperature of the heating element, and to control the maximum safe temperature which the element may attain. An important advantage of such switches resides in the fact that the same may be positioned remotely of the main heating element ii to eliminate undesirable and uncomfortable bulges Within the associated bed Warmer o'r blanket which would be necessitated if direct thermal contact thermostats were scattered, through the Warmer to limit its temperature.

I claim:

1. A thermostatic control for electrical heating elements of variable resistance comprising a first thermally defiectable member, a second thermally deflectable member arranged to move in the same general direction as said first member in response to increased temperature conditions, relatively engageable circuit contacts carried b said members in normally closed relation, said contacts being directly connected in series with an electrical heating element, a lou resistance heating coil carried by one of said members and connected in seriesoupon closure of said contacts with the heating element, and a high resistance heating coil carried by said second member and electrically connected in parallel upon closure of said contacts with the heating element, whereby variations in operating current of the heating element Will result in variation the magnitude of heat energy transmitted by said heating coils to each of said members to cause an uneven thermal den ction thereof and disengagement of said circuit contacts.

2. A thermostatic control for variable resistance electrical heating elements having an operating circuit, comprising a pair of independently supported cooperative thermally defiectable members connected in ser'es with the heating element and arranged to deflect in the same general direction in response to increased temperature conditions, a pair of relatively engageable circuit con-- tacts electrically connected with and carried by said members for directly controlling the ener gization oi the heating element, said thermally defiectable members being arranged so as to normally maintain said contacts in engagement in response to substantially equal temperature conditions at the respective members, a high resistance heater coil carried by one of said members and connected in parallel with the heating element for supplying heat to its associated member, and a low resistance heater coil carried by the other of said members and connected in series with the heating element, whereby a variation in resistance within the heating element causes a variation the magnitude'of heat energy Within said heater coils and an uneven thermal deflection of said members to disengage said contacts.

3. A thermostatic control for variable resistance electrical heating elements having an operating circuit, comprising a first bimetallic arm arranged for thermal deflection in a given plane, a second independently supported bimetallic arm arranged for thermal deflection in the same general plane as said first arm and in the same general direction in response to increased temperature conditions, said arms being electrically connected in series With'the heating element, a pair of relatively engageable circuit contacts car ried by said arms in normally engaged relation, said contacts when engaged serving to close the operating circuit or" the heating element and When disengaged to open the operating circuit or the heating element, a substantially constant resistance heating coil carried by said first arm in series with the heatin element and operable upon of said contacts to heat said first arm, and a high resistance heating coil carried by said second arm in parallel with the heating element and operable upon engagement of said contacts to heat said second arm.

4. A thermostatic control for electrical loads of variable resistance, comprising a pair of relatively engageable overlapping circuit contacts connected in series with the load and operable upon engagement to close the operating circuit of the load, a first bimetal arm supporting one of said contacts and arranged to deflect in a direction to open said contacts in response to increased temperature conditions, a relatively high resistance heating coil carried by said first arm and connected in parallel with the load and operable upon engagement of said contacts to heat said first arm, a second bimetal arm supporting the other of said contacts and arranged to deflect in a direction to close said contacts in response to increased temperature conditions, and a relative- 1y low resistance heating coil carried by said sec- 0nd arm and connected in series with the load operable upon engagement of said contacts to heat said second arm, both of said heating coils being arranged to be'deenergized upon disengagement of said contacts.

5. A thermostatic control for electrical loads of variable resistance, comprising a pair of relatively engageable overlapping circuit contacts connected in series with the load and operable upon engagement to close the operating circuit of the load, a first bimetal arm supporting one of said contacts and arranged to deflect in a direction to open said contacts in response to increased temperature conditions, a relatively high resistance heating coil carried by said first arm and connected in parallel with the load and operable upon engagement of said contacts to heat said first arm, a second bimetal arm supporting the other of said contacts and arranged to defiect in a direction to close said contacts in response to increased temperature conditions, a relatively low resistance heating coil carried by said second arm and connected in series with the load and operable upon engagement of said contacts to heat said second arm, both of said heating coils being arranged to be deenergized upon disengagement of said contacts, and a third bimetal arm linked to one of said first-named bimetal arms and thermally insulated from said heating coils for moving one of said first-named arms independently of thermal deflection within said first and second arms.

6. A thermostatic control for variable resistance heating elements having an operating circuit, comprising a first bimetallic arm having a rigidly supported end and a free end, a second bimetallic arm having a rigidly supported end and a free end arranged to deflect in the path of the free end of said first arm in response to variations in temperature conditions, a pair of relatively engageable circuit contacts carried by the free ends of said arms and connected in series with the heating element, a relatively low resistance heating coil carried in close thermal relation to said second arm and electrically connected in series with the heating element, said low resistance heating coil being operable upon energization to cause said second arm to move in a direction to close said contacts, and a relatively high resistance heating coil carried in close thermal relation with said first arm and electrically connected in parallel with the heating element, said high resistance heating coil being operable upon energization to cause said first arm to move in a direction to open said contacts, both of said heating coils being arranged so as to become deenergized upon disengagement of said contacts. 7. In a thermostatic control for variable resistance electrical heating elements having an operating circuit; a pair of unconnected thermally defiectable members arranged to move in the same general direction in response to increased temperature cond-itions; a pair of relatively engageable circuit contacts carried by said members and directly connected in series with the electrical heating element, said contacts when engaged serving to close the operating circuit of said heating element and when disengaged to open said circuit; a first resistance carried on one of said members and connected in series through said contacts with the heating element for heating one of said members in direct proportion to the amount of current passing to the heating element; and a second relatively higher resistance carried on the other of said members and connected in parallel through said contacts with the heating element for heating the other of said members in proportion to the voltage across said heating element.

8. In a thermostatic control for variable re- 1 sistance electrical heating elements having an operating circuit; a pair of thermally deflectable members arranged to deflect in the same general direction in response to temperature increases and having a pair of normally engaged circuit contacts connected in series with the electrical heating element for directly controlling the energization thereof; means disposed in thermal proximity to one of said arms and connected in series with said heating element through said contacts for supplying heat to said one of said arms in direct proportion to the amount of current passing to said heating element upon engagement of said contacts; and means disposed in thermal proximity to the other of said arms and connected in parallel with said heating element through said contacts for supplying heat to said other of said arms in direct proportion to the voltage across said heating element upon engagement of said contacts.

GEORGE W. CRISE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,693,325 Traver Nov. 27, 1928 1,988,877 Shivers Jan. 22, 1935 2,009,601 Anderson July 30, 1935 2,182,048 Elmer Dec. 5, 1939 2,293,671 Alexander Aug. 18, 1942 2,354,918 Kearsley Aug. 1, 1944 2,445,021 Clark July 13, 1948 

